In the field of thermal generators, a first solution consists in collecting the CO2 present in the combustion fumes prior to discharging them into the atmosphere.
The methods used are generally based on cryogenics, absorption by a chemical or physical means, or on the use of membranes.
The large amounts of fumes to be processed and the low partial pressures of the CO2 in said fumes, which are themselves substantially at the atmospheric pressure, explain why these solutions are however complex and costly to implement.
Other technologies have been provided more recently to limit the final CO2 emissions. Examples thereof are:                conversion of the fuel to hydrogen (H2) and carbon dioxide (CO2) by reforming or partial oxidation prior to the combustion stage.        
This method affords the advantage of increasing the total pressure and the partial pressure of the CO2 in the gaseous products resulting from the reforming stage. This allows thereafter easier separation of the carbon dioxide and of the hydrogen, the hydrogen-enriched and substantially CO2-free gas fraction being then burnt in a gas turbine.
Such a method is for example described in international patent applications WO 99/41,188 and WO 00/18,680.
However, a major drawback of this type of method is linked with the very energy-costly reforming stage, which consequently considerably decreases the overall energy efficiency of a plant working with such a method. Furthermore, the device taken as a whole is bulky, costly and complex to implement because it includes a reforming unit and a separation unit, as well as a thermal generator.                Preliminary separation of the molecular nitrogen (N2) and of the oxygen (O2) present in the combustion air upstream therefrom.        
Combustion is thus carried out in the presence of substantially pure oxygen and the combustion fumes essentially contain CO2 and steam. After recovery of the energy by expansion of the hot fumes, the carbon dioxide is separated from the water, part of the fumes being generally recycled to dilute the oxygen.
A method based on this principle is for example described in international patent application WO 97/07,329, but the economic feasibility of this method is however limited by the high cost of the plant intended for separation of the oxygen and of the molecular nitrogen.
Other methods have also been proposed lately.                International patent application WO 00/57,990 provides, for example, combustion with part of the air from the compressor. The fumes resulting from this combustion then pass through a CO2 absorption device also working under pressure.        
This method has the advantage of increasing the partial pressure of the carbon dioxide and of decreasing the total volume of the gas to be processed in the absorption unit.
However, it involves cooling of the fumes resulting from the combustion (carried out at a temperature generally above 1000° C.) to a temperature ranging between 30 and 120° C. to obtain the conditions allowing said absorption, then heating after this absorption to a temperature ranging between 800 and 900° C. through an exchanger.
Besides the technical difficulties linked with such conditions (notably the realization and the cost of such an exchanger), the overall energy efficiency of the plant is low because of the inevitable thermal losses generated by such a method and of the low temperature of the gases at the expander inlet.                Patent EP 744,987-B1 provides recirculation of part of the fumes (approximately 40%) resulting from the combustion with the combustion air, the other part being sent to a CO2 absorption column.        
This technique allows the level of CO2 present in said fumes to be substantially increased, but this CO2 content increase is however limited.
Furthermore, extraction of the CO2 in the non-recycled fumes fraction is carried out at ambient pressure, therefore with a large gas volume, and it will necessarily lead to cost and space problems.